Sandro Mereghetti, Sergio Campana, Paolo Esposito, Nicola La Palombara, Andrea Tiengo
Stellar evolutionary models predict that most of the early type subdwarf
stars in close binary systems have white dwarf companions. More massive
companions, such as neutron stars or black holes, are also expected in some
cases. The presence of compact stars in these systems can be revealed by the
detection of X-rays powered by accretion of the subdwarf's stellar wind or by
surface thermal emission. Using the Swift satellite, we carried out a
systematic search for X-ray emission from a sample of twelve subdwarf B stars
which, based on optical studies, have been suggested to have degenerate
companions. None of our targets was detected, but the derived upper limits
provide one of the few observational constraints on the stellar winds of early
type subdwarfs. If the presence of neutron star companions is confirmed, our
results constrain the mass loss rates of some of these subdwarf B stars to
values <10^{-13}-10^{-12} Msun/yr.
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http://arxiv.org/abs/1110.5204
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